Calling GAP's StructureDescription from SAGE

Question

Given a sage group object $G$, I want to obtain its structure description using GAP, as follows:

sage:  gap.StructureDescription(G)

The command works fine in sagenb.org, but on my local installation I get an error message like this:

RuntimeError                              Traceback (most recent call last)

/opt/sage-5.3-linux-64bit-ubuntu_12.04.1_lts-x86_64-Linux/local/lib/python2.7/site-packages/sage/interfaces/interface.pyc in __call__(self, *args, **kwds)
...
RuntimeError: Gap produced error output
Error, the Small Groups identification is required but not installed

   executing StructureDescription($sage1);

I was thinking that perhaps I need to install the small groups library. I'm using ubuntu, and in the /usr/local/bin directory I typed:

> sudo sage  -i database_gap-4.4.9

But I still get the error for the gap.StructureDescription(G) command. How do I get this command to work?

Answer 1

This is an example of installing additional GAP components in just installed Sage 5.13 (version numbers may differ in your particular installation, if you have an older version of Sage):

1) Call optional_packages and find names or relevant things (which will hopefully have version numbers matching the core GAP system):

sage: optional_packages()
   ...
  'database_gap-4.6.4',
  ...
  'gap_packages-4.6.4.p1',
  ...

2) Call install_package for those components in which you are interested:

sage: install_package('database_gap-4.6.4')

... lots of output ...

sage: install_package('gap_packages-4.6.4.p1')

... lots of output ...

3) Check how the GAP welcome screen looks like:

sage: gap_console();
 ┌───────┐   GAP, Version 4.6.4 of 04-May-2013 (free software, GPL)
 │  GAP  │   http://www.gap-system.org
 └───────┘   Architecture: x86_64-apple-darwin10.8.0-gcc-default64
 Libs used:  gmp, readline
 Loading the library and packages ...
 Components: trans 1.0, prim 2.1, small* 1.0, id* 1.0
 Packages:   Alnuth 3.0.0, AutPGrp 1.5, CTblLib 1.2.2, FactInt 1.5.3, 
             GAPDoc 1.5.1, LAGUNA 3.6.3, Polycyclic 2.11, TomLib 1.2.2
 Try '?help' for help. See also  '?copyright' and  '?authors'
gap> 

In particular, you may be interested in lines

 Components: trans 1.0, prim 2.1, small* 1.0, id* 1.0

where small* 1.0, id* 1.0 stands for the Small Groups Library, and

 Packages:   Alnuth 3.0.0, AutPGrp 1.5, CTblLib 1.2.2, FactInt 1.5.3, 
             GAPDoc 1.5.1, LAGUNA 3.6.3, Polycyclic 2.11, TomLib 1.2.2

which list some packages that are loaded by default.

4) Now gap.StructureDescription works:

sage: G = PermutationGroup(['(1,2,3)(4,5)', '(3,4)'])
sage: gap.StructureDescription(G)
S5

In addition, it may be useful to check "Can non-isomorphic groups have equal structure descriptions?" from the GAP F.A.Q.

Disclaimers

• The latest version of GAP available for standalone installation may be newer than the one available with Sage

• Not all packages that are redistributed with GAP are available in gap-packages-... component installed as above (there are 114 packages redistributed with GAP 4.7.2), but there is a way to install additional packages manually. On the other hand, Sage includes the braid package, which has been superseded by MapClass which is now an accepted package redistributed with GAP.

Answer 2

I tried the above, also on ubuntu, and got the same error. I believe it is for exactly the reason stated, that the SmallGroups library is needed. From this link http://www.gap-system.org/Manuals//doc/ref/chap76.html#X7B6CD527825945CD it appears that not only do you need to install the package, you most likely need to load it with gap.LoadPackage(name) before you can use it.